Apparatus for imitating thermal conductivity and electrical resistance of diamonds and their substitutes

ABSTRACT

The invention relates to devices which exhibit some physical properties, namely thermal conductivity and electrical resistance of diamonds and their popular imitations, and method for manufacturing and usage of such device. These physical properties of gems are usually used by commercial gem testers for the purpose of distinguishing true diamonds from the fakes. The imitations are made of inexpensive metals, like brass and stainless steel, and conductive plastics, and can replace more costly diamonds and other gems, like moissanite, white sapphire, and others for the purpose of verification of correct operation of gem testing devices.

BACKGROUND OF THE INVENTION

Telling real diamonds from fakes was always the concern of jewelers. A number of commercially available handheld inexpensive devices serve this purpose. Initially it was sufficient to verify high thermal conductivity of a gem in order to assure that it is a diamond. New high tech artificial gems based on silicon carbide possess same high thermal conductivity as diamonds. It is possible to distinguish these from diamonds, which are practically insulators, by their electrical conductivity, which is although small (sometimes very small!) but still measurable. While diamonds do not show noticeable conductivity, artificial carbides have measurable electrical resistance ranging from tens of meg-ohms (10̂6) to hundreds of Giga-ohms (10̂9), even Tera-ohms (10̂12). So, most new diamond testers can measure both, thermal conductivity and high (some more, some less) electrical resistance while applying test voltage of up to 1000 volts.

It is desirable to be able to verify operation of such tester on regular basis, say every day before using it. Such procedure requires having samples of gems, like a diamond, a few moissanites, and other gems and crystals. All these may cost a sizeable sum of money (cost of high quality carbides while less than that of diamonds still significant). Even if it is a justifiable expense, it may not be desirable to entrust such value to personnel working with inexpensive testers. The carbides are not a perfect standard for verifying functionality of a tester because their electrical resistance can vary significantly from one test to another, sometimes by an order of magnitude. Also, they can be sensitive to ambient light, which may reduce electrical resistance significantly. Besides, all necessary gems may simply not be at hand.

SUMMARY OF THE INVENTION

Most commercial diamond testers perform two tests. First, thermal conductivity of the touched object is evaluated. If it is high as expected from real diamonds, electrical conductivity test is performed in order to distinguish diamonds from silicon carbide (moissanite), which has similarly high thermal conductivity but displays measurable electrical conductivity, while diamonds are essentially insulators.

To verify correct operation of such tester one needs a set of objects with verifiable thermal and electrical conductivities. Using a set of gems may be impractical or too expensive, as the testers themselves are not expensive. It is not difficult to simulate required thermal conductivity: silver, copper and its alloys have thermal conductivity high enough for the purpose, while a variety of stainless steel can be used to imitate thermal conductivity of some other diamond substitutes. Such metal pad could be combined with an electrical resistor to address both functions of the tester. The resistor would have to have a value up to hundreds Giga-ohms for this purpose. Although resistors with such values are available they are bulky and expensive.

The present invention addresses this issue by using a tablet of high resistance plastic, of the kind that are produced for anti static packaging and can be made to have specified small electrical conductivity, sandwiched with a disk made of a metal with required thermal conductivity, thus producing a compact and inexpensive part with required combination of physical properties.

This invention suggests that a number of such objects with different combinations of required properties, imitating various diamond fakes and substitutes that are used by jewel industry should be assembled in a compact inexpensive device. The device can be placed in a casing in order to keep the working surfaces clean and dry, as this is essential for correct measurements.

DESCRIPTION OF DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following description of preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 shows the composition, structure, and design of the apparatus.

FIG. 2 shows how such apparatus is used for verification of gem tester correct operation.

DETAILED DESCRIPTION OF THE INVENTION

It is helpful to understand how a diamond tester is normally used. Typical tester looks like item 9 pictured in FIG. 2. It can be held in one hand. If the tester was designed to perform electrical conductivity test, it has conductive pad 10 connected to a source of test voltage inside the device. Person using the tester holds the tester in one hand while touching the pad 10 with his or her finger 20. In the other hand the person holds, say, a metal ring with mounted gem. The probe 11 is designed to sense thermal conductivity of an object it touches, and also conducts electrical current. When the probe 11 touches a gem, the heated probe cools down depending on thermal conductivity of the gem. Electrical portion of the test is done through the circuit formed by the source of test voltage, conductive pad 10, body of the person, metal of the gem setting, the gem itself, and the probe 9. Electrical resistance of the gem is orders of magnitude larger than that of a human body, so that the body can be considered a short circuit in this case. The current through the circuit is very small and poses no danger and causes no discomfort. The tester must be ‘intelligent’ enough to detect when metal setting of the gem (or any object with low electrical resistance) is touched accidentally, and not to apply high test voltage in such case, which otherwise could result in electrical shock to the user.

Centerpiece of the invention is a set of members A through F (FIG. 1) assembled on a base-plate 1 made of a good electrical conductor, e.g. brass, as shown in FIG. 1. Members B through F, to be touched by the tester probe 11 (FIG. 2), provide various combinations of electrical and thermal conductivities as may be necessary to verify correct operation of a diamond tester with various gems. They are described in detail further down. The member A is made of a conductive (metal) post 2 to be touched by a finger 19 of the person holding the tester, through whose body test voltage is conducted from the source inside the tester to the members B through F; it should be wide enough to accommodate a human finger or thumb.

Member B is a conductive post 1 made of metal, and is used to verify that a tester detects correctly when metal gem setting is touched accidentally. This is necessary in order to avoid an electric shock to the user, which could result from applying high test voltage through a low-resistance circuit.

Members C, D, and E each have on top metal disk 4 made of metal with high thermal conductivity as found in diamonds and moissanites. They differ, however, in their electrical resistance. That resistance is determined by the properties of material under the metal disk. Moissanites can have electrical resistance varying widely depending on manufacturing technology. ‘Soft’ moissanites with resistance from tens of Mega-ohms to a few Giga-ohms are detected without applying high test voltage. The member C imitates this variety of diamond substitutes. It uses plastic disk 5 with electrical resistance of a few Giga-ohms. ‘Hard’ moissanites can have electrical resistance reaching Tera-ohms and require high test voltage to measure it. These are imitated by the member D with plastic disk 6 having resistance of hundreds Giga-ohms. Member E imitates diamonds and uses a layer of insulating material (or air) 7 under the metal disk 4.

Member F imitates those diamond substitutes that have low thermal conductivity. It's electrically conductive top layer 8 is made of a metal with low thermal conductivity, e.g. stainless steel, which rests on a layer 7 of insulating material, as such diamond substitutes do not require electrical conductivity test to be performed.

FIG. 2 shows the assembly of FIG. 1 enclosed in casing 12 so that only surfaces 13 through 18 of the members A through F of FIG. 1, which need to be touched by probe 11 in the process of using, are exposed on top of the enclosure 12.

FIG. 2 explains the method of using the described apparatus with a typical diamond tester. While finger 19 of the person doing the tests rests on the exposed surface 13 of member A of FIG. 1 with his(her) other hand holding tester 9 and touching the conductive pad 10 with finger 20, the heated probe 11 of the diamond tester 9 touches exposed surfaces 14 through 18 one at a time in that order. At each touch one operating mode of the tester being evaluated is checked. 

1. A device for imitating diamonds and their common substitutes used in jewelry, in the sense that it exhibits thermal conductivity and electrical resistance of diamonds and their.
 2. Device as in claim 1, which comprises a set of separate members, one imitating diamonds with their high thermal conductivity and very high electrical resistance, another imitating silicon carbide with its high thermal conductivity and small but measurable electrical conductivity, yet another imitating silicon carbide with greater electrical conductivity, yet another for those diamond imitations which possess low thermal conductivity, and, finally, an electrically conductive member for verifying ability of the gem tester to detect when gem setting, like a ring, is touched by the probe accidentally instead of the gem.
 3. Device as in claim 1, with members, which have desirable combination of thermal and electrical properties, made in a sandwich form with: top layer made of metal or alloy, which is touched by thermal-electrical sensor of a gem tester and has required thermal conductivity and a very small electrical resistance; middle layer made of material with required electrical resistance; and bottom layer made of a good conductor through which the test voltage is applied.
 4. Device as in claim 1, where the members, which imitate gems with high thermal conductivity, like diamond and silicon carbides, has a top layer made of any material, most likely metal or alloy, with high thermal conductivity and very low electrical resistance.
 5. Device as in claim 1, where the member, which imitates diamond fakes with low thermal conductivity has the top layer made of any material, most likely metal or alloy, with sufficiently low thermal conductivity and very low electrical resistance, e.g. stainless steel, which can be made to have specified thermal conductivity.
 6. Device as in claim 1, where the member which imitates diamonds has the middle layer made of a good insulator, e.g. insulating plastic or air.
 7. Device as in claim 1, where the members, which imitate gems with measurable electrical conductivity, have their middle layer made of anti-static plastics, like Durathane, or other high resistivity material, which can be made to have required electrical resistance; two different grades are used for the members imitating high and low resistance silicon carbide crystals.
 8. Device as in claim 1, where the member, which imitates diamond fakes with low thermal conductivity that do not require electrical testing has the middle layer made of a good insulator, e.g. insulating plastic or air.
 9. Device as in claim 1, which is equipped with a member made of metal for applying test voltage via a finger of the person using it, or any other means for connecting to a test voltage source.
 10. Device as in claim 1, where all members are mounted on an electrically conductive base, typically made of metal, through which test voltage can be delivered to those members of the device whose electrical resistance must be evaluated in the course of a test, namely the conductive pad imitating metal and the middle layers of the members imitating diamond substitutes with measurable electrical conductivity.
 11. Device as in claim 1, where all members are assembled in a compact block in a piece of electrically insulating material with very low thermal conductivity.
 12. Device as in claim 1, where the block of members as in claim 11 is placed in a protective casing, to prevent accumulation of moisture or other foreign substance on its surface, which could cause electrical leakage across the surface.
 13. A method for simulating thermal conductivity and electrical resistance of materials by using three substitute materials, each exhibiting one of the required properties, assembled in a layered structure as in claim
 3. 14. A method for verifying performance of gem and diamond testers, which employ measurement of thermal and electrical conductivities, using the apparatus as in claim
 1. 